1. Field of the Invention
The present invention relates to the field of non-volatile memory devices. More particularly, the invention relates to a multi-bit flash electrically erasable programmable read only memory (EEPROM) cell with a bitline.
2. Discussion of Related Art
Memory devices for non-volatile storage of information are currently in widespread use today, being used in a myriad of applications. A few examples of non-volatile semiconductor memory include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash EEPROM.
Semiconductor EEPROM devices involve more complex processing and testing procedures than ROM, but have the advantage of electrical programming and erasing. Using EEPROM devices in circuitry permits in-circuit erasing and reprogramming of the device, a feat not possible with conventional EPROM memory. Flash EEPROMs are similar to EEPROMs in that memory cells can be programmed (i.e., written) and erased electrically but with the additional ability of erasing all memory cells at once, hence the term flash EEPROM.
An example of a single transistor Oxide-Nitrogen-Oxide (ONO) EEPROM device is disclosed in the technical article entitled xe2x80x9cA True Single-Transistor Oxide-Nitride-Oxide EEPROM Device,xe2x80x9d T. Y. Chan, K. K. Young and Chenming Hu, IEEE Electron Device Letters, March 1987. The memory cell is programmed by hot electron injection and the injected charges are stored in the oxide-nitride oxide (ONO) layer of the device. Other examples of ONO EEPROM devices are disclosed in U.S. Pat. Nos. 5,635,415; 5,768,192 and PCT patent application publication WO 99/07000, the contents of each reference are hereby incorporated herein by reference.
In the case of known NROM devices, such as schematically shown in FIG. 1, an NROM cell 100 included a grid of polygates or word lines 102 and buried bitlines 104. The bitlines 104 were formed in the N+region of the substrate so that a higher density of bitlines can be formed that region versus when the bitlines were formed in a metal layer. As shown in FIG. 2, the junction formed between the buried bitlines 104 and the substrate had a severe differential change going from the bitline region to the substrate region which caused the capacitance of the junction to be large. It is well known that the higher the capacitance of the junction, the more the read speed is slowed down. Select transistors 106 were required to compensate for the high capacitance and the slowing of the read speed. The overall size of the flash memory cell array increases the more select transistors 106 that are employed in the array.
One aspect of the invention regards a process for forming an array of memory cells that includes forming a buried bitline region by implanting an n-type dopant in a region of a semiconductor substrate, wherein there is a severe differential change going from the bitline region to the substrate region causing the capacitance of a junction between the bitline region and the semiconductor to be large and reducing the capacitance of the junction.
The above aspect of the present invention provides the advantage of decreasing the junction capacitance and decreasing the number of select transistors needed in a memory cell array.
The above aspect of the present invention provides the advantage of reducing the total size of a memory array.
The present invention, together with attendant objects and advantages, will be best understood with reference to the detailed description below in connection with the attached drawings.